The invention relates to a retort assembly for the Kroll reduction of a metal in chloride form with a reducing metal.
Titanium, zirconium and hafnium are commercially produced by the reduction of these metals in chloride form (from upstream processing steps) with a reducing metal, such as with magnesium, to form a metallic sponge disc. The sponge disc is typically consolidated and further processed. In the production of zirconium, for example, zirconium tetrachloride powder is placed in the annular chamber of a reduction retort having a centrally located, downwardly facing vapor flow pipe. Metallic magnesium is placed in a crucible and the crucible is then welded or otherwise suitably attached to the bottom of the retort with the vapor flow pipe extending downwardly into the crucible. The assembly is then evacuated and its contents brought up to process temperature to vaporize the zirconium tetrachloride and to melt the magnesium. The vapor in the retort is then permitted to flow through the vapor flow pipe and into the crucible where it reacts with the liquid magnesium to form zirconium and magnesium chloride.
The product from the above Kroll reduction process is a disc-shaped solid which generally comprises two fairly well defined portions. A cup-shaped portion comprised of zirconium which generally takes the shape of the crucible is commonly referred to as the "sponge" or as the "sponge disc". A second portion comprised of magnesium chloride extends downwardly into the cup-shaped portion. The disc-shaped solid develops such a configuration because the downwardly flowing zirconium tetrachloride vapors generate sufficient turbulence in the central portion of the liquid magnesium in the crucible to entrain the zirconium particles which are formed and thereby retard their deposition in the center. The zirconium particles are circulated to less turbulent peripheral portions of the liquid in the crucible where they form a skull against the walls of the crucible and a sponge disc which develops inwardly from the skull.
The skull (which may be 25 to 40 millimeters thick) tends to weld with crucible walls and remain in the crucible when the solid disc is mechanically removed from the crucible. Although such a skull represents an obvious product loss, it is generally considered to be an unavoidable loss because the skull is contaminated by iron from the crucible walls (or from a crucible liner). Even with the formation of a skull, the sponge disc from the above described Kroll process may have a high iron content.